Biodiesel has physico-chemical properties that are similar to those of petroleum-based diesel. Because of its renewability, biodiesel has attracted tremendous amount of interests from oil and chemical companies as well as newly emerged alternative fuel companies. The conventional process used for biodiesel production is cost intensive, which is partly attributed to the transesterification reaction (Bautista et al., 2009). In addition, despite numerous environmental benefits compared with petroleum-based diesel, the conventional biodiesel production also has some environmental challenges. For example, methanol, which is routinely used in the transesterification reaction, can be hazardous. When removing residual triglycerides and glycerol from the biodiesel product, multiple steps of water wash produce massive industrial wastewater that can have tremendous negative impact on the environment. Therefore, novel strategies for biodiesel production are highly sought by the industry.
Biodiesel is a mixture of fatty acid methyl esters (FAMEs) that are derived from a variety of crop oils, animal fats or waste oils. In the conventional process of biodiesel production, transesterification is a critical step. It utilizes basic or acid catalysts to convert triglycerides into FAMEs in the presence of methanol with glycerol as a byproduct. The existence of glycerol has been proved to affect the quality of biodiesel, such as viscosity, flash point and oxidation stability (Tan et al., 2010). Therefore, glycerol has to be removed.
In energy crops for biodiesel production, fatty acids are synthesized then condensed with glycerol to form triglycerides. This industrial practice has also led to the excessive supply of glycerol and its devaluation in the market price (Lu et al., 2008). The transesterification reaction itself also has a number of technical challenges, such as the low reaction rate when using the acid catalyst and the formation of soap in basic-based process. There have been numerous attempts focusing on improving the conventional transesterification process for biodiesel production. For example, several studies reported utilizing lipase as a catalyst to catalyze the removal of glycerol and the formation of FAMEs (Sanchez and Vasudevan, 2006). In a recent report, methyl acetate, instead of methanol, was used in the transesterification reaction in order to reduce the influence of glycerol (Tan et al., 2010). While some progress has been made in improvement of biodiesel production, break-though concepts and technologies still need to be developed in order to significantly lower the cost of biodiesel production to make it economically feasible.